An image sensor is generally a semiconductor device generating image data by sensing a light beam reflected from an object. Such an image sensor includes a micro-lens array that guides the incident light beam toward a photodiode to generate photoelectric charges corresponding to the amount of the incident light beam received.
With reference to FIG. 1, which illustrates a schematic cross-sectional partial view of an exemplary complementary metal oxide semiconductor (CMOS) image sensor taught in prior art. The illustrated CMOS image sensor 10 includes a semiconductor structure, which has: a substrate 101 with the property of photodiode; a light receiving surface 102 on the substrate 101; a dielectric layer 103 having a metal wiring layer 104 therein and being above the light receiving surface 102; a color filter layer 105 on the dielectric layer 103; a micro-lens layer 106 on the color filter layer 105; and a cap oxide layer 107 on the micro-lens layer 106. As shown in FIG. 1, the use of the cap oxide layer 107 is to improve sensitivity of the CMOS image sensor 10 because it provides a larger light collecting surface. For instance, an incident light beam L1 is through the cap oxide layer 107, the micro-lens layer 106, the color filter layer 105, and the dielectric layer 103, but is blocked by the metal wiring layer 104; the incident light beam L1 is continuously reflected by the metal wiring layer 104 in order to become an eject light beam L2; further, another pair of an incident light beam L6 and an eject light beam L7 is the same in behavior and properties as the pair of the incident light beam L1 and the eject light beam L2. Another incident light beam L3 can be directly transmitted through the cap oxide layer 107, the micro-lens layer 106, the color filter layer 105, the gaps between the metal wiring layer 104, the dielectric layer 103, and the light receiving surface 102 to the substrate 101; further, other two incident light beams L4 and L5 are the same as the incident light beam L3. As it can be seen, the convex curvature and thickness of the cap oxide layer 107 increase the light collecting surface, so that the cap oxide layer 107 is capable of collecting more light beams. Accordingly, the cap oxide layer 107 may be easily cracked or peeled off due to unstable stress occurring at the interface between the cap oxide layer 107 and the micro-lens layer 106.
Therefore, there is a need in providing an image sensor capable of preventing a cap oxide layer thereof from cracking or peeling off, thereby improving the reliability and lengthening the lifetime of the image sensor.